Sheathed electric heating elements, and particularly range surface units, of the general type to which this invention relates, are well known in the art. For illustrative examples of some units of the prior art, reference is made to U.S. Pat. No. 2,508,552, issued May 23, 1950, to W. R. Tuttle, and U.S. Pat. No. 2,550,843, issued May 1, 1951, to J. McOrlly.
Over the ensuing years, many refinements have been made to increase the efficiency of such prior art units, and millions have been made and sold because of their recognized ability to function in acceptable manner.
However, one thing that has troubled the industry prior to my invention is the manner in which units of this type fail, if and when they ultimately fail. It is well known to those in the industry that metal sheath range surface units of prior art design reach an end of life in an unpredictable mode of failure. Some units may fail in a passive manner with nothing more than a parting of the resistor alloy to create an open circuit. Other units may fail with a breakdown of insulation resistance to the grounded metal sheath, and this breakdown causes an electrical arc to be generated which melts the resistor alloy and metal sheath similar to the arc of an electric arc welder.
The energy created by the arc is sometimes sufficient to cause molten metal of fusion to be expelled in a violent manner, so as to burn holes in the bottom of cooking utensils, but more seriously, to shower molten metal particles about the area. While damage from this violent type of failure has usually been minimal, nevertheless there exists the potential of fire or bodily injury.
My invention minimizes or completely eliminates the hazards above-mentioned through changes in the internal design of the range surface unit. In the improved form, the usual helical resistance coil is embedded within refractory material which is in compacted condition within the metal sheath. A return conductor is disposed within the coil and is electrically connected to one end of the resistance coil, and extends through the latter to the other end thereof where it and the other coil ends are connected to terminal conductors for connection to a source of electrical energy.